1. Field of the Invention
This invention relates to a technique for making a circuit substrate, such as a package for mounting a semiconductor device or chip. More particularly, it relates to a transfer sheet for forming an exact circuit pattern, and a process using such a transfer sheet for easily making a circuit substrate, particularly a circuit substrate having a three-dimensional electrical circuit.
2. Description of the Related Art
Conventionally, several kinds of circuit substrates are known in which a circuit pattern is formed on a resin base, such as a printed circuit board (PCB) or a pin-grid array (PGA) type package for mounting a semiconductor chip, and to form such a circuit pattern on the resin base, several methods such as plating, heat transfer, or the like are known.
A conventional heat transfer method, as disclosed for example, in Japanese Unexamined Patent Publication No. 60-121791, comprises the steps of: forming a circuit pattern of conductive paste on a base sheet by a screen-printing method; to form a transfer sheet on which the circuit pattern has been coated with an adhesive layer, and placing the transfer sheet in an injection mold; and thereafter injecting a heatproof, thermoplastic melting resin into a cavity of the injection mold to obtain a resin base having the circuit pattern formed thereon; and then peeling the base sheet from the resin base.
In another method of forming a circuit pattern on the base sheet, a metal film is first formed thereon by vaporization and then photoetched, to obtain a desired circuit pattern.
The above-mentioned method using a transfer sheet has the following disadvantages. Namely, it is difficult to form a precise circuit pattern by a screen-printing method. If the circuit pattern is formed of a conductive paste which consists of metal particles joined together with an organic binder, the resistance of the circuit is increased, and although a good heatproof material adapted for use as the resin base has been developed, such as an engineering plastic including glass fillers, the organic binder included in the conductive paste does not have good heatproof characteristics, and thus a circuit substrate thus obtained has a poor heat resistance.
On the other hand, if a circuit pattern is formed by photoetching the vaporized metal film, it is possible to obtain a precise pattern, but it takes a relatively long time to obtain a thick vaporized metal film, and a circuit having good wire-bonding characteristics can not be obtained if the vaporized metal film is thin.